Integrated Pest Management

IPM Manual for Structural Pests in British Columbia

Chapter 3: Ants

Learning Objectives

When you have completed this lesson, you should be able to:

1. Describe characteristics of carpenter ants, pharaoh ants, odorous house ants, cornfield ants and thatching ants.
2. Describe colony development and propagation for each of these groups of ants.
3. Describe the damage or problems caused by each of these groups of ants.
4. Describe where to look for carpenter ant nests, pharaoh ant nests and cornfield ant nests.
5. Describe preventative measures that can be used to deter populations of carpenter ants, pharaoh ants, odorous house ants, cornfield ants and thatching ants.
6. Describe pesticides preferred for use in IPM programs to control carpenter ants, pharaoh ants, odorous house ants, cornfield ants and thatching ants.

Ants are common structural pests in British Columbia. The importance of different species varies with their geographical location in the province. This chapter gives a detailed look at the five groups of ants most commonly encountered by pest managers in British Columbia:

• carpenter ants
• pharaoh ants
• odorous house ants
• cornfield ants
• thatching ants

Biology of Ants

Ants are social insects and live in large colonies. There are two main types of ants in a colony: queens and workers. The queen ant lays eggs, while worker ants care for the larvae, forage for food or defend the nest. In carpenter ant nests, the worker ants vary in size, depending on the role they play in the nest. This can make identification difficult. 

Ant species differ in the number of queens per colony, how and what time of year the colony reproduces and the type of food they prefer. It is important to know these characteristics to plan an effective control program, therefore it is essential to identify problem ants correctly.

Depending on the species, there may be one queen or many queens to a colony. Since the key to eliminating an ant colony is to kill the queen or queens, the pest manager must know how many queens to expect in a colony (see Table 1 for information on common ants).

Depending on the species, ant colonies multiply by swarming or budding; they may also extend their colonies by developing satellite nests. Swarming means that at certain times of the year winged males and winged females leave the colony to mate. The mated females, or queens, then establish new colonies.

Budding behavior in ant colonies means that a group of workers leaves the colony. They carry larvae and pupae away to start a nest in a new location. The group may include a queen (from species that have colonies with multiple queens) but a queen is not necessary to start the new colony.

Satellite colonies or nests, such as those made by carpenter ants, are separate nests that act as extensions of the main colony. They remain in contact with the main nest as worker ants go back and forth. Older larvae, pupae and workers are found in satellite colonies but not queens, eggs or the youngest larvae.

Ants have a complex food cycle, in which food is exchanged between worker ants and larvae. Food may be passed from one individual to others many times in a nest. When the workers feed on a slow acting insecticidal bait, such as sugar mixed with boric acid, they live long enough to feed the poison to the larvae. In this way the poison is spread throughout the colony.

Carpenter Ants

Identification

Ten species of carpenter ants are native to British Columbia; the most important pest species on the West Coast is a big black ant, with the scientific name Campanotus modoc. A black ant with a reddish, middle section, called Campanotus vicinus, is more common in drier areas.

Carpenter ants cannot be reliably identified by size or colour because they are so variable. Carpenter ant workers have a single bump (or node) on the narrow stem between the middle body segment (or thorax) and the final body segment (abdomen). They also have a smoothly rounded side profile to the thorax, with no dip or "sway-backed" appearance (see figure).

Carpenter ants do not eat wood, but they do excavate into wood to expand their nests. They can also build nests inside walls without excavating wood. When they chew into wood, the damage is characterized by smooth galleries that run with the grain of the wood. It may be confused with damage caused by wood boring beetles, cornfield ants, and dampwood or subterranean termites.

Monitoring

Careful visual inspections are necessary to find the main nest(s). Inspections should start with a search for worker ants foraging for food. It has been estimated that about 10% of the workers are out foraging at any one time. These foraging ants can often be followed back to the main nest along their trail. This is sometimes difficult to do because their trails can run along electrical wires and pipes inside walls. Carpenter ants are most active at night so it may be necessary to inspect then if the nest proves hard to find.

The main nest is always in a permanently moist area, particularly where wood is in contact with soil. About 80% of the time, the main nests for carpenter ant colonies are outdoors, such as in rotting stumps, the heartwood of old trees, in landscape wood, including railroad ties (the ants are not deterred by creosote treatments) and fence posts.

Satellite nests, which are extensions of the main colony, are commonly found inside structures, although they can also be outdoors. Satellite nests can be inside walls, in ceilings, under outdoor siding, along drain spouts near the roof line, around roof gutter braces, under roofing panels, in hollow doors, in floors, in window sills, under bathroom floors and in ceiling insulation. There can be several satellite nests as far away from the main colony as 200 m. Clues to finding satellite nests are:

• frass and sawdust falling out of cracks and crevices in walls, around electrical boxes,
• sawdust piles or insect parts in attics,
• rustling noises in the wall, and
• places where vegetation or other objects act as a bridge from the ground to the structure.
  

Action Decisions

The presence of carpenter ants foraging for food inside a building does not necessarily mean the structure has an infestation. They may be coming from a colony outside the building. The presence of these foraging trails, however, with ants carrying food into a building, is an indicator of an indoor nest. The presence of carpenter ants indoors between January and April may also indicate an indoor nest.

Depending on the situation, a large numbers of carpenter ants, more than 10 at a time, found indoors at any time of the year would be an action threshold for treatment.

Prevention

Make Repairs

Eliminating sources of moisture and repair rotting wood. Look for any site where wood is in contact with soil and repair or remove such things as:

• leaking roofs
• blocked gutters
• inadequately vented crawl spaces
• rotting decks
• leaking plumbing
• soil piled above the sill plate
• sub-floors over concrete in contact with soil
• landscaping wood
• old firewood
• rotting logs, stumps, fence posts
• live trees with rotting centres
• vegetation too close to buildings

Barriers

Slippery barriers of Teflon® can be applied to foundations to prevent ants from climbing foundation walls. These barriers work best when applied to smooth concrete walls just below the building siding and at least 30 cm above ground level. Corners of buildings are difficult to protect with slippery barriers.

The need for outdoor perimeter sprays can be reduced, but not necessarily eliminated, by the Teflon barriers. The barriers do not kill ants, but can be used to force ants to travel only on sprayed surfaces.

Controls

Remove Nests

Once found, a nest must be removed quickly because the ants will begin to scatter as soon as the nest is disturbed. They can move larvae and pupae to safety in a few minutes. A vacuum is very useful in quickly removing ants. A thorough job involves removing all infested wood, insulation or other materials and as many ants as possible.

Preferred Pesticides

Dusts: Carpenter ant satellite nests in dry locations can be eliminated with dusts such as diatomaceous earth, silica aerogel (e.g., Drione^®) or boric acid. The dust is usually blown into the wall voids through small holes drilled in the wall. Dusts must be applied very thinly (using fine nozzle dusters), because thick layers of dust repel insects. Insert the nozzle of the duster around and behind the electrical plugs, switch boxes, and past insulation. Do the same along enclosed plumbing where possible. This is important because ants use wires and plumbing in walls as travel routes through the house. Boric acid and sorptive dusts do not degrade inside wall voids, therefore they provide long lasting protection against carpenter ants and other insects.

Where physically removing nests is impractical, sprays or dusts containing pyrethrins or boric acid will work if they can be applied directly into the nest. Diatomaceous earth will also work if the nest site is dry.

Baits: A granular Abamectin carpenter ant bait was registered for indoor use in 2005. Abamectin is an extract from the naturally occurring soil microorganism Streptomyces avermitilis. It may be useful as part of an integrated pest management program but is unlikely to be the only control necessary. Other ant baits currently available will not eliminate colonies of local carpenter ant species.

Other Pesticides

High pressure sprays of building exteriors are of no benefit in protecting buildings from carpenter ant infestations and should not be used.

Residual insecticides around the foundation perimeter of a structure may be required especially where building are in wooded areas. Carpenter ant nests are naturally numerous in forest areas and are a continuous source of re-infestation in buildings.

Perimeter sprays are applied around the foundation of a structure to prevent carpenter ants from entering. Perimeter sprays of permethrin (e.g. Prelude or Dragnet) are registered for this use outdoors. These formulations are effective outdoors for about 2 - 4 weeks. It is usually sufficient to apply the pesticide in a continuous thin line along sill plates and/or along the edge of the siding.

Perimeter sprays should be done in the spring or early summer when carpenter ants are actively searching for new locations for satellite nests.

Future Controls

Heat Treatment: In the United States an experimental patented process uses hot forced air to control termites, carpenter ants and other structural pests. Special equipment and training is needed to raise the temperature in infested homes safely to a temperature lethal to insects.

Evaluation

Post-treatment inspections are essential to determine the success of the treatment(s). If many foraging ants are present after treatments, then the main colonies may not have been eliminated.

Regular inspections will be needed to check on effectiveness of perimeter pesticide applications and to reapply as needed.

Pharaoh Ants

Identification

Pharaoh ants (scientific name, Monomorium pharaonis) are small (3 mm long) yellow, ants. They have two obvious bumps (nodes) on the narrow stem between the thorax and the abdomen.

They prefer warm, moist conditions and frequent areas with high bacterial concentrations.

Pharaoh ant colonies are very large, containing up to 300,000 individuals and several queens. Pharaoh ant colonies reproduce solely by budding. Budding activity can be triggered by overcrowding in the nest, by disturbing the nest or by insecticide applications.

Monitoring

Visual inspections are necessary to find all areas of ant activity. Nests can be in any void or space, including walls, boxes, folds of sheets, stacks of paper, above and below cabinets and inside food storage cabinets. Colonies are usually found inside buildings, in warm places, especially near a source of water. Due to their budding behaviour, colonies may be numerous and widespread.

Pharaoh ants leave visible foraging trails and use wiring and pipes inside walls as foraging routes. It is estimated that only about 3%-5% of the pharaoh ant population is out searching for food at a time. It is more important to find all areas where the ants are actively looking for food than to find all of the nests. This can be done by setting out non-toxic food baits to locate as many areas of ant activity a possible ( see Box) and using this information to determine where to place insecticide baits.

Pharaoh Ant Survey

The survey uses non-toxic baits, such as mint-apple jelly to attract the ants. Bait stations consist of a dab of jelly on small (2-3 cm) squares of cardboard or pieces of masking tape, placed wherever the ants are most likely to be found. Particular attention should be paid to areas near dish washing equipment, water fountains, aquariums, houseplants and around windows where moisture condenses.

Record the location of each survey bait station on a detailed map of the building or site. Check for ant activity around the bait stations and record the foraging activity on the map. This map can then be used as a guide for placing toxic baits.

Action Decisions

Pharaoh ants do not cause structural damage to buildings but they can spread disease. Even a small infestation should be eliminated in hospitals and other medical facilities.

Prevention

Sanitation is an important aspect of pharaoh ant control. Food should be stored in glass or plastic containers with tight fitting lids. Garbage should be removed daily.

It is very important to remedy moist conditions and remove sources of moisture by making needed repairs.

Controls

Preferred Pesticides

Baits: Using insecticidal baits is the recommended method to control pharaoh ants. Boric acid (e.g.,Drax^®) hydramethylnon and abamectin baits are registered for this use. These slow-acting baits do no cause the colonies to spread by budding. By the time the colony begins to die off, it is too late for the workers to start a new colony.

To make sure foraging ants from all colonies in the building take up the toxic bait, bait stations should be placed in all areas of ant activity found in inspections and through the survey. Bait stations should be placed in nearby areas as well.

Eliminating pharaoh ants with baits can take several months. Sanitation is essential to remove other possible food sources so that ants are forced to eat the bait. Frequent monitoring during the baiting program is very important to locate new areas of ant activity and to replace insecticidal baits as they are eaten.

Other Pesticides

Insecticidal sprays or dusts should not be used. These are rarely effective and usually make the problem worse because they cause the ant colonies to bud, resulting in an increasing number of more widely scattered colonies.

Evaluation

Continue to check the success of the treatment by observing the bait stations weekly. Eventually there should be no ants at the bait stations and none actively foraging in the building.

Odorous House Ants

Identification

Odorous house ants (scientific name, Tapinoma sessile) are small black ants (2.4 to 3.3 mm long) often found in kitchens in the early spring. They feed on a wide variety of foods but seem to be especially attracted to sweets such as spilled juice and jam. When crushed they emit a distinct odour (like rotten coconuts, according to one source), which can be used to help identify them. Colonies can be very large. In large infestations, there are usually several nest sites, which are all part of the same colony.

Monitoring

Visual inspections are necessary to find the nest(s).

Nests are frequently in wall or ceiling voids and can be outside under stones, firewood piles, or under building foundations.

Odorous house ants follow each other in distinct foraging trails. Usually a few minutes of observation will enable the observer to identify the ants' pathways. Outdoor areas, basement and attic areas close to indoor pathways should be checked for ant activity.

Action Decisions

Odorous house ants frequently live in homes without becoming a nuisance. Those found indoors in the early spring may stop their activity indoors once warm summer weather arrives or when their preferred food source is removed. In these cases, no special action is needed to control them.

If the ants are routinely foraging indoors and no action is taken, they will likely recur in subsequent years. As the colony grows over the years, the ants may become more of a nuisance and control measures may become advisable.

Prevention

Sanitation: Odorous house ants are usually a problem when they find food in homes. To eliminate their food supply, store sweet foods in clean containers with tight-fitting lids, remove garbage often (daily if possible) and clean up food spills thoroughly and promptly.

Exclusion: These ants are most often pests in kitchens, which they enter through cracks around moldings and plumbing, in foundations or exterior walls. These cracks should be carefully sealed.

Control

Preferred Pesticides

Baits: Odorous house ants can usually be eliminated with insecticidal baits containing boric acid, hydramethylnon or abamectin. The bait stations should be placed directly in their foraging trails. Sometimes using a combination of two different types of bait at the same time gives better control than a single type of bait. Repeated bait applications are usually needed to eliminate colonies. Weekly monitoring of the infestation is essential so that baits can be replaced or moved as required until control is achieved.

Dusts: Diatomaceous earth, boric acid dusts or pyrethrins can be applied into the cracks and crevices to keep ants out of problem areas. Pyrethrins may also be applied as a spray. These pesticides can also be applied to nest sites to eliminate the colonies.

Evaluation

Continue inspections to find out whether control measures have been effective.

Cornfield Ants

Identification

Cornfield ants (a confusing group of Lasius species), are a problem in homes in the lower mainland, the Fraser Valley and on Vancouver Island.

Cornfield ants may be confused with carpenter ants. Both can nest in rotting wood and females of both are large and have wings. Cornfield ant workers and males are considerably smaller (2 to 2.5 mm long) than carpenter ant workers and males. The females and workers of the cornfield ant are tan, whereas carpenter ants are all black or black with a red thorax. When seen from the side view, the middle section, or thorax, of the cornfield ant has a notch or dip in it while the profile of the carpenter ant's thorax is smoothly rounded.

Monitoring

Visual inspections are necessary to find the nest(s).

Cornfield ants prefer moist areas and colonies are usually found in decayed logs, stumps. or in the soil. They may be in flower boxes, under cement slab foundations and in basements. In homes, colonies usually infest decayed wood that has been previously damaged by carpenter ants or termites.

Although cornfield ants are secondary pests of already decayed wood, they can speed the deterioration of wood. Cornfield ants make galleries, which are quite different from carpenter ant galleries.

Action Decisions

When a cornfield ant colony is found in the home, this is an indication of moisture problems and decaying wood. Action should be taken to find and repair the damage as this will give long-term control.

Prevention

Make Repairs: New colonies will inevitably invade if the moisture and decayed wood are not removed. Repair moisture problems and replace decaying wood. It may be necessary to vent bathrooms, seal cracks in cement foundations, and seal porches and steps where they join exterior walls. In instances where rotting wood is under a slab, sealing the cracks through which the ants emerge can provide sufficient control.

Control

Preferred Pesticides

Baits: Ant baits made with sugar as the attractant can sometimes be effective in controlling cornfield ants.

Other Pesticides

Other pesticides for control of cornfield ants are almost never needed and will only provide temporary control unless underlying moisture problems are corrected.

Evaluation

As with other ants, check to see if control measures have been effective.

Thatching Ants

Identification

There are many different species of thatching ants (a group of Formica species) in British Columbia. Several are red and black, while others are all black. Worker ants range in size from 3 to 9 mm in length, depending on the species. Winged male ants of some species look almost identical to winged male carpenter ants. Winged females are smaller than winged carpenter ant females.

The common species of thatching ants nest in rotting wood and the surrounding soil. Some species build nest mounds of evergreen needles and twigs. Occasionally satellite nests are built in wall voids, attics and other parts of buildings. One species that is often a structural pest nests in rotting wood and rock cavities in sunny locations.

There can be several queens in the colonies and colonies can contain many thousands of workers. Mating flights usually occur in the late summer.

Monitoring

Visual inspections are necessary to find the nest(s).

It is usually easy to find, and follow, thatching ants to their feeding sites and nest sites during daylight hours. Night-time inspections may be needed in problem situations.

Action Decisions

Whether control is required depends largely on the species of thatching ant present.

One species of thatching ant builds enormous colonies with nests that spread over many square metres of ground. This species is often aggressive and workers readily bite. When large colonies of these ants are found in or around homes, control is definitely needed.

Another species of thatching ants usually nests in obvious mounds with well-defined feeding trails. In this situation, the need for control will depend on where the nest is, where the ants are searching for food and how easy it is for people to avoid the ant trails.

A third species of thatching ant is very common and found in small numbers in most yards. They rarely bite, and are rarely present in large enough numbers to warrant control.

Prevention

Removing rotting stumps on the property will eliminate many potential thatching ant nest sites.

To prevent thatching ants from moving into buildings, repair leaks, ensure adequate ventilation of all areas and ensure that no wood is in contact with soil.

Controls

Preferred Pesticides

Baits: Thatching ants can sometimes be eliminated with baits. (delete words here) Often a combination of bait types works best. Repeated bait applications are usually needed to eliminate the colony.

Dusts: Thatching ant nests in buildings can usually be eliminated with boric acid, diatomaceous earth or pyrethrins. These can also be applied to cracks and crevices used by the ants as travel routes into problem areas. In addition to eliminating them in buildings, however, the ants should be followed to find other nests outdoors.

Other Pesticides

Thatching ant nests can also be eliminated using spot treatments of nests with residual insecticides registered for ant control. There is no need to apply residual insecticides to large areas outdoors.

Evaluation

Follow-up inspections should be conducted to make sure control measures have been effective.

Further Reading

Bennett, G., J. M. Owens, R. M. Corrigan. 1988. Truman's Scientific Guide to Pest Control Operations. 4th ed. Purdue University/Edgell Communications. 495 pp.

Hansen, L.D. 1993. Carpenter Ants and Urban Pest Management. Pest Control Technology. 21(3):52-25.

Hedges. S. A. 1992. PCT Field Guide for the Management of Structure-Infesting Ants. Franzak & Foster Co., Cleveland OH. 155 pp.

Mallis, A. 1990. Handbook of Pest Control. 7th ed. Franzak and Foster, Cleveland, OH. 1,152 pp.

Olkowski, W., S. Daar and H. Olkowski. 1991. Common-Sense Pest Control. The Taunton Press, Newtown, CT. 715 pp.